It is known that X-ray changes its intensity and phase by interaction between the X-ray and an object when the X-ray is incident on the object. The phase of the X-ray interacts with the object more strongly than the intensity of the X-ray does. Researches have been conducted actively on X-ray phase imaging that uses the above property of the X-ray to get a high contrast image (hereinafter referred to as phase contrast image) of an object with low X-ray absorption properties based on the phase change (angular change) of the X-ray caused by the object.
An X-ray imaging system using the Talbot effect caused by two transmission-type diffraction gratings (grids) is devised as one type of X-ray phase imaging (for example, Japanese Patent No. 4608679 and C. David et al., “Differential X-ray phase contrast imaging using a shearing interferometer”, Applied Physics Letters, Vol. 81, No. 17, October 2002, page 3287). In this X-ray imaging system, a first grid is disposed behind an object when viewed from an X-ray source, and a second grid is disposed downstream of the first grid by the Talbot length. Behind the second grid, an X-ray image detector for detecting the X-ray to generate an image is disposed. Each of the first and second grids has a stripe pattern of X-ray absorbing sections and X-ray transmitting sections extending in one direction and the absorbing sections and the X-ray transmitting sections are alternately arranged in a direction orthogonal to the extending direction. The Talbot length is a distance at which the X-ray passed through the first grid forms a self image (fringe image) due to the Talbot effect. The fringe image formed by the Talbot effect is modulated by an interaction (phase change) between the object and the X-ray.
In the above X-ray imaging system, moiré fringes generated by the superposition (intensity modulation) of the self image of the first grid and the second grid are detected using a fringe scanning method. Phase information of the object is obtained from changes in moiré fringes caused by the object. In the fringe scanning method, images are captured while the second grid is translationally moved in a direction substantially parallel to the plane of the first grid and substantially vertical to a grid direction of the first grid at a scanning pitch that is one of equally-divided parts of a grid pitch, and then angular distribution (differential image of the phase shift) of the X-ray refracted by the object is obtained from a change in each pixel value obtained by the X-ray image detector. Based on the angular distribution, the phase contrast image of the object is obtained. The phase scanning method is used in an imaging apparatus using laser light (for example, see Hector Canabal et al., “Improved phase-shifting method for automatic processing of moiré deflectograms”, Applied Optics, Vol. 37, No. 26, September 1998, page 6227.)
The first and second grids require high X-ray absorption properties. In particular, the second grid requires higher X-ray absorption properties than the first grid to surely provide intensity modulation to the fringe image. For this reason, the X-ray absorbing sections of the first and second grids are formed with gold (Au) with a large atomic weight. The X-ray absorbing section of the second grid requires a large thickness in an X-ray traveling direction relative to its width, that is, a so-called high aspect ratio. The aspect ratio is a value obtained by dividing the thickness of the X-ray absorbing section by the width of the X-ray absorbing section. The above-described second grid has a microstructure. For example, the pitch between the X-ray absorbing sections is several μm, and the thickness of the X-ray absorbing section is several tens to several hundreds of μm in the X-ray traveling direction.
In the Japanese Patent No. 4608679, methods for producing first and second grids are disclosed. In one of the producing methods, grooves are formed using photolithography on a photosensitive polymer layer provided on a metal seed layer on a substrate. Au is deposited in the grooves by electroplating using the metal seed layer as an electrode. In another producing method, on one of faces of a plate-like silicon layer with the thickness of 50 μm, a seed layer is made of titanium or silicon using vapor deposition. The Au is deposited in the grooves, formed by etching the plate-like silicon layer, by electroplating using the seed layer as the electrode.
U.S. Patent Application Publication No. 2010/0278297 discloses that, as a conventional technique, a grating with grating webs and grating gaps alternately and periodically arranged is provided with filler beams for connecting the adjacent grating webs to provide stability to a grid structure. The filler beams are provided randomly along an extending direction of the grating gaps. The grating webs correspond to the X-ray absorbing sections, and the grating gaps correspond to the X-ray transmitting sections. The U.S. Patent Application Publication No. 2010/0278297 discloses that randomly provided filler beams generate capillary force acting in the grating gaps to bend the grating webs, and that an interval between the filler beams needs to satisfy a predetermined geometric condition in the extending direction of the grating gaps to prevent the grating webs from bending.
In the method for producing the grid disclosed in the Japanese Patent No. 4608679, a photosensitive polymer layer is used to form the X-ray absorbing section. This method is susceptible to the influence of the photosensitive polymer in processing accuracy, so the grid cannot be produced with high accuracy. The grooves are formed through the synchrotron radiation exposure with high directivity and development. Because polymer is soft, grid patterns such as plate-like patterns standing upright on a substrate are likely to be deformed by sticking (a phenomenon in which adjacent patterns stick to each other) caused by swinging or vibration of a solution during the development or surface tension of water during drying. Thus, it is difficult to maintain the uniformity in width and height of the grid with high accuracy. Because the Au has higher rigidity than the polymer, the polymer is likely to be deformed depending on the growth of the Au plating. This significantly degrades the performance of the grid. In addition, there are few domestic facilities capable of performing synchrotron radiation exposure. The exposure takes a long time, resulting in a low throughput, and thus it is not suitable for manufacture. Instead of using the photosensitive polymer layer, a silicon layer may be formed on the substrate. However, it is technically difficult to form the silicon layer by coating similar to that for the photosensitive polymer layer because it is necessary to melt the silicon at a temperature of at least 1400° C.
In another producing method disclosed in Japanese Patent No. 4608679, groove sections are formed by etching the thin-plate silicon layer with the thickness of 50 μm. Normally, the lower limit of the thickness of the silicon substrate is of the order of 200 μm to allow ease of handling including transfer to the etching device. Even if a titanium layer or a silicon layer is formed using the vapor deposition, the thickness of the formed layer is of the order of 1 μm, so the layer does not reinforce the plate-like silicon layer. Accordingly, it is unrealistic to form the grooves on the 50 μm plate-like silicon layer by etching. The titanium or silicon layer with the thickness of the order of 1 μm may be in a state of floating inside the groove section of the plate-like silicon layer. It is easily expected that the titanium or silicon layer comes off in the subsequent steps, for example, in the electroplating step. Thus, it is difficult to perform the electroplating inside the grooves.
In the U.S. Patent Application Publication No. 2010/0278297, the filler beams are effective for reinforcing the structure of the grating webs because the filler beams connect the grating webs corresponding to the X-ray absorbing sections. The filler beams, however, are not effective in preventing the photosensitive polymer layers from sticking when grating webs are formed using a method disclosed in, for example, Japanese Patent No. 4608679.
An object of the present invention is to provide a grid having X-ray absorbing sections with a high aspect ratio and a method for producing the grid with high accuracy.